1. Field of the Invention
This invention relates to an automatic image focusing device for optical instruments such as photographic cameras, and more particularly to a device for automatically focusing an image focused by an objective in a single lens reflex camera or the like.
2. Description of the Prior Art
It has been known in the art to automatically focus an image in a photographic camera. In most of the conventional automatic image focusing device, a photodetector is located substantially on the image plane of an object of the optical instrument like a camera and the objective is moved back and forth along the optical axis thereof to find the extreme value of the output of the photodetector. When the focusing plane or the image plane of the objective is aligned with the photodetector, the output of the photodetector is maximized. The output of the photodetector is transmitted to a lens control means and the axial position of the objective is controlled to focus an image on the photodetector in accordance with the output of the photodetector.
The above described automatic image focusing device has a defect in that the variation in the output of the photodetector is slow and the extreme value thereof must be detected and accordingly the focus control cannot be quickly conducted. Particularly, since the extreme output cannot be detected unless the objective is moved back and forth around the focusing position to find the extreme value, it is impossible to know whether the image is best focused at a properly selected position.